It is known that variations in a concentration of a pulp suspension are of decisive importance for the screening process. A reduction in the concentration implies an increase of the hydraulic load on the screen means, i.e. the flow rate through the orifices in the screen increases. At concentrations below about 0.5% the capacity becomes unacceptably low. An increase in the concentration implies an increase of the energy intensity required for breaking up the fiber network into individual fibers and causes it to become fluid, so-called fluidization, which is a prerequisite for the screening process. The concentration, therefore, sets a limit for efficient utilization of the screen. As a result of too high a concentration, the flocks of the pulp will not be broken up, which implies that the screening process cannot continue.
In a conventional pressurized screen for pulp suspensions, thickening along the length of the screening zone is the physical problem which limits the effectiveness of the screen with regard to both capacity and efficiency. From a physical point of view, the thickening implies that the concentration of the fiber suspension increases from the inlet to the reject outlet along the surface of the screen basket. Increased concentration implies that die strength of the fiber network increases considerably.
Due to the fact that the rotary part of the screen rotates at equal speed along the entire length of the screening zone, the energy supply is substantially constant from the inject end to the reject end of the screen. This implies that the screening must start at too low, a concentration at the beginning of the screening zone, in order to prevent the pulp concentration from rapidly becoming so high that a large portion of the screening zone operates as a thickener. An energy intensity too high in relation to the pulp concentration implies that the fiber suspension is overfluidized at the beginning of the screening zone, which yields an unnecessarily high turbulence level and, as a result, deteriorated separation selectivity. After a short zone with ideal conditions, the pulp concentration will be too high, the energy will no longer be sufficient for breaking up the fiber network, and the final portion of the screening zone operates as a thickener. In other words, the thickening implies that the screen looses efficiency and capacity.
In connection with certain modern pulp screens, one has increased the pulp concentration by providing inside the screen a rotor with pulsation-creating wings, which yield an extended suction pulse producing a vacuum adjacent to the screen, in order to recover a certain amount of the liquid lost by the thickening. At the same time, an overpressure arises on the inside of the pulsation wings. Extended suction pulses by the use of wide pulsation wines renders it possible to increase the concentration in a screen, but at this high a concentration the process, according to the above reasoning, becomes very sensitive from the point of view of optimization. Small variations in the pulp concentration, dewatering properties or fiber length distribution affect the critical balance between network strength and energy supply. As a result, one is forced to operate the screen at a number of revolutions higher than at optimum, in order to manage the operability even during normal process variations. Especially at the end of the screening zone, the effect of the suction pulses tends to diminish, with thickening problems resulting.
In accordance with the present invention, these and other difficulties have been overcome by the invention of apparatus for screening fiber suspensions comprising a housing, an annular screen disposed within the housing, a rotor having an outer surface defining a first end and a second end disposed within the annular screen for rotating relative to the annular screen, pulsation wings disposed between the outer surface of the rotor and the annular screen for relative rotation with the rotor whereby a screening zone having a first end and a second end is formed between the rotor and the annular screen, an inlet for the fiber suspension for feeding the fiber suspension to the first end of the screening zone, an accept outlet for a portion of the fiber suspension which passes through the annular screen, a reject outlet for a portion of the fiber suspension which does not pass through the annular screen, and a dilution liquid nozzle for supplying a dilution liquid to the screening zone, the dilution liquid nozzle disposed proximate to the reject outlet and directing the dilution liquid into the screening zone along the outer surface of the rotor. In a preferred embodiment, the pulsation wings include a first end and a second end and the dilution liquid nozzle is disposed at a height corresponding to the second end of the pulsation wings.
In accordance with one embodiment of the apparatus of the present invention, the dilution liquid nozzle comprises a plurality of the dilution liquid nozzles located circumferentially around the outer surface of the second end of the rotor. Preferably, the apparatus includes a stationary wall member disposed proximate to the second end of the screening zone adjacent to the second end of the rotor, the stationary wall member comprising a portion of the dilution liquid nozzle and forming a gap seal with the second end of the rotor.
In accordance with another embodiment of the apparatus of the present invention, the dilution liquid nozzle includes a gap for the dilution liquid, and the apparatus includes a stationary wall member disposed proximate to the second end of the screening zone adjacent to the second end of the rotor and forming a gap seal with the second end of the rotor, the gap being partially defined by the stationary wall member. In a preferred embodiment, the gap is defined by a space between the stationary wall member and the second end of the rotor.
According to the present invention, the above problems can be reduced substantially by forming the screen with a dilution liquid supply at the reject outlet by means of nozzles directed into the screening zone. The variations in thickening arising under normal conditions can thereby be counteracted effectively, especially during changes in production and quality. It is then possible to increase the ingoing and outgoing concentration of the screen, its capacity and efficiency, and to lower the energy consumption. The dilution liquid supply according to the present invention has the essential advantage that it permits one to drive the screen at a lower number of revolutions.